1. Field of the Invention
The invention relates to wear-through detection in multilayered parts, and, more particularly, to wear-through detection in semiconductor vacuum processing systems.
2. Description of the Related Art
The use of ion implantation equipment to introduce conductivity-altering dopants into semiconductor wafers has become an integral part of the fabrication of semiconductor devices. A simplified schematic of a representative known ion implanter system 100 is shown in FIG. 1A. An ion source 1 generates positively charged dopant particles by known means which are directed as an ion beam 2 along a beam path toward a target 10, typically a semiconductor wafer, housed within target chamber 7. A manipulator 3 having extraction electrode 12, ground electrode 13, and source exit aperture 14, extracts and initiates the travel of the ion beam 2 along the beam path. Along the beam path, the ion beam 2 is deflected and focused by mass analyzer 4. The mass analyzer 4 uses magnetic forces to select ions having desired mass and charge from undesired ions. The ion beam is focused in the plane of an aperture 5 as a mass resolving assembly. As known in the art, different ion species in ion beam 2 are deflected through different angles by the mass analyzer 4. A desired ion species passes through the aperture 5 to target 10, while undesired ion species are intercepted by the walls 5' of aperture 5. The ion beam 2 passing through aperture 5 is then accelerated to a desired energy by an accelerator 6, such as a high voltage coil, and is incident on the target 10 located within target chamber 7.
As known, the ion beam 2 can be distributed over the surface of the target 10 by mechanically scanning target 10 with respect to the beam, or vice versa, or a combination of these techniques. For instance, the workpiece target 10 can be moved relative to the ion beam through mechanical or electrostatic means. This ensures that the implant is done uniformly across the workpiece 10, and that the workpiece 10 does not overheat from the high power density being delivered by the beam. The beam 2 typically is directed to a beamstop area (not shown) when the workpieces 10 are not being implanted. The workpiece 10 must be overscanned; i.e., the beam 2 usually goes fully off the workpiece 10 during each scan. In this case, the beam 2 travels beyond the workpiece 10 and strikes a graphite plate, where it is stopped. The target chamber 7 also can include a known system for automatically loading semiconductor wafers into one or more target positions. The entire region between the ion source 1 and the target 10 is maintained at high vacuum during ion implantation.
The ion implanter equipment 100 has inner structural surfaces that are exposed to, and subject to wear, by the high energy ion beams 2. These inner surfaces of the ion implanter are usually made of metallic materials, such as stainless steel or aluminum. Stainless steel, for instance, contains iron, nickel and chromium constituents, which, as with the aluminum, can contaminate the semiconductor devices if these inner surfaces are eroded, and the removed material is transported to the target and implanted into the semiconductor devices. The consequences of this contamination of the wafer are very severe. The resulting semiconductor devices will have degraded performance, reliability, and functionality, such as having high device leakage and defective oxides.
As a conventional approach to dealing with this problem, inner surfaces of the ion implanter apparatus 100 that are known to be subject to ion beam erosion have been covered with graphite, which in small quantities does not seriously affect semiconductor device performance. For instance, the ion beam is rastered across many different parts of the beamline during beam tuning and beam set-up. These parts usually are graphite-coated metal parts, such as graphite-coated stainless steel. The graphite coating has varying thickness, depending on the location in the vacuum processing tool, generally averaging about 1/4 inch (about 6.4 mm) in thickness. These graphite-coated metal tool parts are designed to be replaced periodically, viz., before the graphite is worn away and underlying metal is exposed, and becomes eroded or sputtered. However, if this periodic change-out does not occur as scheduled, or a defect is missed during routine maintenance inspections for erosion, or an anomalous wear rate occurs (e.g., due to species mix or liner material variation), or new areas become exposed due to subtle changes in beam column alignment, magnetic fields, new species, or vacuum leaks, then the metal atoms in the metal underyling the graphite coating can become eroded away by the ion beam and become contaminants within the system.
A specific known application of a graphite-coated tool part in the ion implantation system 100 involves the aperture or other apertures used along the beam path, such as aperture 5. The aperture 5 is in the form of a solid plate having an opening Q coincident with the beam line. With reference to FIG. 1B, such an aperture 5 has been made of an aluminum or stainless steel substrate plate 5b covered with graphite 5a to prevent sputtering of the metal 5b by the ion beam 2. Only the part of the beam 2 that is aligned with the aperture opening Q will be passed on down the beamline of the implanter, and the remainder of the beam 2 impacts the graphite plate 5a and is blocked. Again, sputtering of the outer graphite coating 5a in small quantities poses little risk of contamination problems to the semiconductor devices being formed in the wafer. However, as shown in FIG. 1C, over time, the outer graphite covering 5a eventually is worn away by high energy ion beams used for processing within the tool 100 to expose the underlying metal substrate 5b of the aperture 5. Once exposed, the underlying metal 5b is undesirably sputtered and its constituents volatized and implanted into the wafer 10 raising the risk of failures in the chip devices. Also, as seen in FIG. 1C, the size of opening Q in the eroded aperture 5 tends to increase with time as it is eroded by the beam 2. This causes the beam to increase in cross-sectional area 11, which is undesirable.
Another specific problem area occurs in the mass analyzer 4. As shown in more detail in FIG. 2, the ion beam 2 enters a region of high magnetic field "M", caused by the analyzer magnet 4. This field region M exerts a force tangential to the direction of travel of a given ion. The ions are displaced from their straight-line path in an amount proportional to their charge, proportional to the magnetic field, and inversely proportional to their molecular weight. The magnetic field strength is adjusted so that the path of the desired species is curved a given amount from the original direction of travel. The inner chamber surface 18 of the mass analyzer 4 has been covered with a thin graphite (or silicon) liner 15 and a striker plate 16 also having a graphite liner 15 has been used on the outer wall 9 of the mass analyzer 4. A conventional arrangement of magnets, not shown, are located with magnetic cores facing the chamber walls 9, 18 of mass analyzer 4. The beam of the desired species is directed through aperture 5, previously discussed in connection with FIG. 1A, which is small enough such that only the desired ionic species is transmitted through it. The desired ion beam is, after passing analyzer exit aperture 5, very pure chemically and has a very tight energy distribution. The direction of the ion beam is fine-tuned through the use of quadruple electrostatic lenses 8 located downstream of accelerator 6. The ion beam 2 then enters the target process chamber 7, as discussed above, where the actual workpiece to be implanted is held. Here again, the graphite liner coatings 15 provided on the inner chamber walls of the mass analyzer 4 can become eroded completely through by action of the ion beam 2 to allow undesired erosion of the underlying steel substrate surfaces.
A residual gas analyzer (RGA) 17, as indicated in FIG. 1A, has been used to detect vacuum system problems or outgassing of particular constituents from a workpiece layer of an implantation (end) chamber of an ion implantation system. A residual gas analyzer is a piece of equipment that withdraws gas from a vessel and ionizes the withdrawn gas sample. The resulting ionized beam is accelerated, and run through a mass analyzer. The mass analyzer involves a region of high magnetic field, the strength of which is adjustable by adjusting the electric current through coils of the magnet. The trajectory of the ion beam being analyzed is curved by the magnetic field, in proportion to the field strength and the charge to mass ratio of the ion. The ion beam is then directed towards an aperture plate, similar to that found in an ion implanter. The portion of the beam curves so that it is aligned with the centerline of the aperture, passes through the aperture, and all other portions of the beam are blocked from traveling further. The intensity of the beam at this given magnetic field setting is measured. A spectrum, or distribution of intensity versus charge to mass ratio, can be obtained by varying the magnetic field strength. The composition of the gas in the vessel can be determined by analyzing this spectrum, and comparing it to the mass/charge ratio of known materials. RGAs are used to detect leaks, and analyze processes. For example, as a known practice, if abnormal amounts of H.sub.2 O, O.sub.2, and N.sub.2 are detected, in ratios proportional to those of normal air, then a vacuum leak is suspected. Also, helium has been applied to the exterior of the vacuum processing tool with a suspected leak. If there was a leak, the helium is drawn into the vacuum chamber, and the RGA would then quickly signal the presence of high amounts of helium in the vacuum chamber, signifying that there is a leak in the suspect region of the equipment. An RGA has also been used for troubleshooting vacuum and process problems, by showing the presence of abnormal materials, relative to normal operation, and the type and quantity of these abnormalities. Also, RGA's have been used for end point detection in dry etch tools.
However, there currently is no real-time method for determining if and when the graphite coatings or other protective tool part liners provided on vacuum processing tool parts have become worn through.
Numerous other techniques have been used to analyze constituents of plasmas, and of materials in vapor or gaseous form. These include various forms of spectroscopy, where emitted light (as from plasma) or transmitted light (as through a gas or other fluid) are analyzed and correlated to the absorption or emittance spectra of known materials. The presence of certain wavelengths indicates the presence of a given material, the intensity at that wavelength is related to the concentration of that material, UV, visible light, and IR spectra are commonly used to characterize constituents of plasmas, gases, vapors, and liquids.